The present invention relates to a method for producing a gas component detecting element and a detector.
Hitherto, it has been known to use a gas component detecting element comprising titanium dioxide for indirect detection of air-fuel ratio of internal-combustion engine. This known method utilizes the property of titanium dioxide to change its electrical resistance depending on concentration of the gas component. In this way the concentration of the gas component is detected by the change of electrical resistance, and air-fuel ratio of internal-combustion engine is indirectly detected based on the detected concentration of gas component.
In the case of this gas component detecting element comprising titanium dioxide, titanium dioxide alone is slow in absorption and resorption of gas component and so low in sensitivity and therefore, conventionally a platinum/rhodium catalyst was supported on the surface of titanium dioxide particles for enhancement of sensitivity (e.g., Japanese Patent Kokai No. 56-112638).
Internal-combustion engines are used at temperatures in a very wide range (200.degree.-1000.degree. C.). Therefore, particles of the platinum/rhodium catalyst easily move about on the surface of titanium dioxide particles and they gather and agglomerate to result in growth of particles due especially to the heat under high temperature.
As a result, catalytically active areas on the surface of titanium dioxide particles decrease and absorption and resorption of gas component on the surface of titanium dioxide particles become slow to cause reduction of sensitivity for gas component.
The inventors have conducted intensive research on these points and have found the cause for movement of catalyst particles under high temperature. That is, it is postulated that catalyst particles firmly bond to surface defects (called active points) of titanium dioxide particles with bonding of S.M.S.I. (Strong Metal Support Interaction) as disclosed, for example, in "American Chemical Society", pages 2870-2874 published in U.S.A. in 1979. The surface defect of titanium dioxide means lattice defect caused by escaping, from the crystal lattice, of unpaired electrons (dangling bond), oxygen or the like on the surface of titanium dioxide. Bonding to such surface defects result in strong adherence of the surface of titanium dioxide particles and catalyst particles.
However, the surface defects (active points) of titanium dioxide particles disappear owing to reduction of specific surface area accompanied by growth of particles, and bonding of defects to each other and rearrangement of defects by the heat at heat treatment, namely, calcination which is a pretreatment for stably obtaining the final desired product, namely, titanium dioxide sintered body (for reducing heat shrink and obtaining desired density). Thus, the surface defect density markedly decreases.
As a result, even if catalyst particles are supported on titanium dioxide particles having a low surface defect density, a proportion of these catalyst particles which bond to the surface defects is very small and thus movement of catalyst particles is brought about due to heat.
The present invention aims at inhibition of particle growth by preventing movement of catalyst particles.